1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer or an electrophotographic copier, in which paper is conveyed in a circulatory conveyance path for formation of images on both sides of the paper.
2. Description of the Related Art
Image forming apparatuses are known which comprise circulatory conveyance means (hereinafter referred to as an ADU) for reversibly conveying paper on which an image has been formed on one side of the paper and circulatorily conveying the paper in order to allow images to be formed on both sides of the paper such as standard paper.
Among these image forming apparatuses, there is known a type of image forming apparatus employing a so-called non-stacked circulatory conveyance method in which a plurality of sheets of paper (e.g., five sheets of paper) are continuously supplied from a paper supply tray, a toner image formed on a photosensitive drum is continuously transferred onto a first or front side of the paper and the toner image is fixed onto the paper. Thereafter, the paper is reversed by the ADU, and circulatorily conveyed towards the transfer region where another toner image formed on the photosensitive drum is transferred onto a second or back side of the paper, fixed to the paper, and then the paper is discharged to a discharge tray, so that plural sheets of paper are not stacked at a time in an intermediate tray.
FIG. 1 shows, in an illustrative view, an example of the structure of such image forming apparatus.
A copier as an example of such image forming apparatus comprises an automatic document feeder 1 on an upper part of the body of the copier, and includes within the copier body an image reader 2, an image formation section 3, a paper housing section 4, a paper supply section 5, a reverse/discharge/paper resupply section 6, and an ADU 8 that serves as the circulatory conveyance means.
The automatic document feeder 1 is a device for sending out documents one by one at a time, conveying the documents to an image reading position, and discharging documents whose image has been read to a predetermined place. The automatic document feeder 1 includes a document mount 11 on which documents are mounted; document separation means 12 for separating the documents mounted on the document mount 11; document conveying means 13 including plural rollers for conveying the documents separated by the document separating means 12; document discharge means 14 for discharging the documents conveyed by the document conveying means 13; a document discharge tray 15 for receiving the documents discharged by the document discharge means 14; and document reversal means 16 comprising a pair of rollers for reversing the documents to read images on both sides of the documents. Plural sheets of the documents (not illustrated) mounted on the document mount 11 are separated one by one at a time by the document separation means 12, and conveyed toward the image reading position by the document conveying means 13.
The image reader 2 is disposed below the document conveyance means 13, and images on the documents are read through a slit 21 forming the image reader 2. Documents whose images have been read are discharged onto the document discharge tray 15 by the document discharge means 14.
When images on both sides of a document are to be read, documents for which an image on one side has been read are guided to the document reversal means 16, the documents are reversed by counter-rotational control of the rollers in a state in which trailing ends of the documents are held between the rollers, and then the documents are conveyed once more by the document conveyance means 13, whereby it is possible to read the images on the other sides of the documents at the image reading position. This process is repeated for the number of documents mounted on the document mount 11.
The automatic document feeder 1 is structured to be retractable. By retracting the automatic document feeder 1, a document can be mounted directly on a platen glass 22 and copied.
The image reader 2 is a means for obtaining image data by reading the images on the documents. The image reader 2 includes the slit 21; a first mirror unit 23 comprising a lamp 231 as a light source that irradiates the documents, and a first mirror 232 that reflects light reflected from the documents; a second mirror unit 24 comprising a second mirror 241 for reflecting light from the first mirror 232 and a third mirror 242; an image formation lens 25 that images onto an image pickup device 26 such as CCD light reflected from the second mirror unit 24; and a line CCD 26 adapted to obtain image information by photoelectrically converting the light image imaged by the image formation lens 25. After the image information thus obtained is subjected to appropriate image processing, it is temporarily stored in a memory not shown.
When the documents fed by the automatic document feeder 1 are read by the image reader 2, the first mirror unit 23 and the second mirror unit 24 are positioned as illustrated.
Images on documents that are directly mounted onto the platen glass 22 are read by the first mirror unit 23 and the second mirror unit 24 which are moved along the platen glass 22 while maintaining an optical path length therebetween.
The image formation section 3 forms images by an electrophotographic process on the basis of the image data obtained by the image reader 2. The image formation section 3 includes the photosensitive drum 31 having on a surface thereof a photoelectrically conductive photosensitive layer; an electrifier 32 for uniformly electrifying the surface of the photosensitive drum 31; a laser writing system 33 as an exposure means for exposing the photosensitive drum 31 to form a latent image thereon that is activated on the basis of the image-processed image data; a developer 34 for developing the latent image formed on the photosensitive drum 31 to form a toner image; a transfer electrode 35 for transferring the toner image onto the paper; a charge remover 36 for accelerating separation of the paper with the toner image transferred thereon from the photosensitive drum 31 by effecting an AC corona discharge to remove the charge; cleaning means 37 for cleaning the photosensitive drum 31 after the transferring process has been concluded; a fixing device 38 of heating roller type for fixing the toner image on the paper; and the reverse/discharge/paper resupply section 6 and the ADU 8.
The photosensitive drum 31, which is rotated in a direction shown by an arrow by appropriate drive means, is sequentially electrified by the electrifier 32, the electrostatic latent image is formed by dot exposure by the laser writing system 33, and the toner image is formed by the developer 34. Then, the toner image is transferred via the action of the transfer electrode 35 onto the paper that is synchronously conveyed by initiating rotation of registration rollers 56, which are second paper supply means, so that the paper is superposed on the toner image region, whereby an image is formed on the paper.
The paper with the toner image transferred thereon is separated from the photosensitive drum 31 by the charge remover 36, and the toner image is fixed on the paper by the heat and pressure applied by the fixing device 38.
The photosensitive drum 31 continues to rotate while the remaining toner on the photosensitive drum 31 which has passed through the transfer region is removed by the cleaning means 37 for preparation of the next image formation.
A conveyor belt 39 for conveying the paper separated from the photosensitive drum 31 by the charge remover 36 is disposed between the charge remover 36 and the fixing device 38.
Paper feed trays 400, 410 and 420 including housing sections 405, 415 and 425 comprising housing containers for housing stacked sheets of the paper and the paper supply units 51, 52 and 53 that serve as first paper supply means are arranged one on another in the paper housing section 4. Sheets of paper of different size are respectively housed in these paper feed trays 400, 410 and 420.
The paper supply units 51, 52 and 53 include paper separation rollers 506, 536 and 556 for preventing overlapping supply of paper and paper supply rollers 505, 535 and 555 which are positioned at substantially fixed positions with respect to the paper feed trays loaded in position in predetermined positions in the apparatus.
The paper supply section 5 includes pairs of conveying roller R1, R2, R3, R4, R5 and R6 as conveying means for conveying the paper from the respective paper feed trays to the image formation section 3. PS is a photosensor which detects whether or not paper that has been fed from the paper feed trays has arrived at the positions of the conveying roller R1, R2 and R3 disposed downstream of the separation rollers.
The reverse/discharge/paper resupply section 6 is a region for discharging the imaged paper or resupplying the imaged paper in accordance with a required copying process. The reverse/discharge/paper resupply section 6 includes switching means 62 for switching the conveying path depending upon the situation that (1) the paper discharged by the fixing/discharge rollers 61 is to be discharged as it is to the outside of the apparatus, (2) the paper is to be discharged after it has been reversed, or (3) the paper is resupplied in order to form an image on the back side of the paper.
If it is required that the imaged paper is to be discharged with the final image formed on its top side, the switching means 62 is positioned as indicated by one-dotted chain line in the drawing. On the other hand, if it is required that the imaged paper is to be reversed to form images on both sides of the paper and then discharged, the switching means 62 is positioned as indicated by a solid line in the drawing, the paper conveyed by the fixing/discharge rollers 61 is once conveyed toward the ADU 8. Having passed through the switching means 62, the paper is reversibly conveyed by the conveying rollers 600 so that it passes through the left side of the switching means 62, and is discharged by the discharge rollers 63 onto the discharge tray 64 outside of the apparatus.
Moreover, if it is required that an image is to be formed on the back side of the paper as well, the switching means 62 is positioned as indicated by the solid line in the drawing, the paper conveyed by the fixing/discharge rollers 61 is conveyed as far as the ADU reversal rollers 800 by each of the conveying rollers of the reverse/discharge/paper resupply section 6 driven by a discharge motor, the paper is reversed by switching back by means of these rollers, and the paper is circulatorily conveyed to the registration rollers 56.
In the above structure, the ADU reversal rollers 800 are driven forwards and backwards by an ADU reversal motor M2 shown in FIG. 2, the conveying rollers 810, 820, 830, 840 and 850 are driven by a conveying motor M2 (as shown in FIG. 2) as a driving source, and the registration rollers 56 are driven by a registration motor M5 shown in FIG. 2.
The following operations will occur in a case where, for example, five sheets of paper are taken as a set and continuous image formation is effected by using the non-stacked circulatory conveyance method.
To describe the process by which image are formed on both sides using the ADU 8 while noting the motion of the paper, the five sheets of the paper, which are continuously fed at every predetermined time from the paper feed tray 40 in response to the implementation of the image formation process, reach the registration rollers 56 via the conveying rollers 55, are detected by a detection means not shown, resupplied by the rotation of the registration rollers 56, and enter the transfer region so that each paper is superposed on the toner image region formed on the photosensitive drum 31.
The toner image is transferred onto the first side of the paper by the action of the transfer electrode 35 in the transfer region and the paper is separated from the photosensitive drum 31 and conveyed toward the fixing device 38.
The paper with the toner image fixed thereon by the action of heat and pressure in the fixing device 38, is conveyed downward along the conveyance path that has been switched by the fixing/discharge rollers 61 and the switching means 62, is sequentially moved along the conveyance path formed toward the upper right direction by the conveyance rollers 600, 610 and 620, and thereafter reaches the ADU reversal rollers 800.
The ADU reversal rollers 800 are driven to rotate in the opposite direction at a timing at which the rear edge of the paper is nipped between the ADU reversal rollers 800 in the traveling direction. Therefore, the paper is reversed and conveyed on the conveyance path formed by the ADU 8, i.e., horizontally towards the right of the drawing at the conveying rollers 810 through 850, while the paper is conveyed in the opposite direction with its previous rear edge being as a new front edge. Thereafter, the paper is guided upward and sequentially arrive at the registration rollers 56 after passing through the conveying rollers 55, and a toner image is formed on the second side of the paper in the same process that has been described previously.
After the toner image on the second side of the paper is fixed by the fixing device 38, the paper is sequentially discharged onto the discharge tray 64 via the fixing/discharge rollers 61.
In this kind of image forming apparatus, a predetermined paper size (in particular, a length of paper in its travelling direction) is used as a standard, and a number of sheets of paper with an image formed thereon obtainable per unit of time is set for various paper size (feeding length). In order to achieve this, a distance from the exposure section to the transfer electrode 35 and a distance from the registration rollers 56 to the transfer electrode 35 are set to be equal to each other so that the paper of standard size reaches the registration rollers 56, the process for forming a toner image on the photosensitive drum 31 is initiated at a timing of supply of the paper by the registration rollers 56 and the paper is superposed on the toner image region at a transfer region of the transfer electrode 35. The registration rollers 56 are always activated at fixed intervals by setting linear velocities of the photosensitive drum 31, the registration rollers 56 and the transfer rollers 57 to be the same. Various operative conditions such as a length of the circulatory conveyance path of the paper are set so that a distance between consecutive papers with images formed on their first sides can be the same as a distance between the last paper that will just have an image formed on its first side and the first paper that will have another image formed on its second side by controlling the registration rollers 56 to be activated at a predetermined interval of time. With this structure, high productivity per unit of time that is a number of papers with images formed on both sides thereof that can be obtained per unit of time (minute) (hereinafter referred to as CPM) is realized.
In the image forming apparatus with the above described, any advance or delay for paper to arrive at the registration rollers 56 may occur after an image has been transferred on the paper on its first side and fixed until the paper is reversed for formation of another image on its second side thereof. One of causes for such unstable movement of the paper is that the paper is conveyed only by the conveyor belt 39 after the paper is separated from the photosensitive drum 31 until it is nipped by the pair of rollers of the fixing device 38. Therefore the paper may be moved out of place forward and backward on the conveyor belt 39 during conveyance. Another cause is that the ADU reversal rollers 800 may slip when the paper is reversed thereby in the circulatory conveyance path.
There has been proposed a method to solve this problem in which the paper is conveyed to the position of the registration rollers 56 at an earlier time and is caused to wait there until the scheduled time to adjust the advance or delay of the paper.
Another method for compensating the advance or delay of the paper is disclosed in Japanese unexamined patent publication No. 8-231133 in which conveying rollers are stopped temporarily to cease the paper depending upon the condition of the paper while being conveyed in the circulatory conveyance path so that the advance or delay of the paper is adjusted before it arrives at the registration rollers. However, some of the conveying rollers must be stopped while others need not be stopped depending upon the size of the paper, so that a conveying mechanism including the rollers and control are complicated. This will result in high cost of the apparatus and causes a difficulty in down sizing of the apparatus.
In some kinds of apparatus, conveying velocity of the paper is controlled to be accelerated and then decelerated during conveyance in order to realize high CPM (For example, Japanese unexamined patent publication No. 58-182655).
The present invention was made in the light of the above-described circumstances, and it is an object of the present invention to provide an image forming apparatus of high CPM in which paper is circulatorily conveyed without being temporarily stopped.
In order to achieve this object, according to the first aspect of the present invention, there is provided an image forming apparatus comprising first paper supply means for supplying paper one sheet at a time; second paper supply means for receiving the paper from the first paper supply means and conveying the paper to an image formation section; the image formation section for forming an image by fixing a toner image after the toner image has been transferred to the paper supplied from the second paper supply means; and circulatory conveyance means for reversibly conveying and circulatorily conveying the paper, on which the image has been formed on a first surface thereof, to the second paper supply means once again, in order to form an image on a second surface of the paper, on which the image has been formed on the first surface thereof by the image formation section; after an image has been sequentially formed on the first surfaces of a predetermined number of sheets of the paper by the image formation section, the sheets of paper being reversibly conveyed and circulatorily conveyed to the second paper supply means by the circulatory conveyance means, and then image being successively formed on the second surfaces of the paper, on which the image has been formed on the first surfaces thereof; wherein a position at which the paper, on which the image has been formed on the first surface thereof, is accelerated from the paper conveyance speed during image formation in order for reversal and a position at which the post-reversal paper conveyance speed is decelerated in order to form the image on the second surface of the paper are alterable; and wherein images are formed on both sides of the paper without effecting control of the operation of the second paper supply means restricted by the number of image formed sheets per unit of time.
In the above aspect, the position or the timing at which the paper is accelerated for reversal may be determined by measuring the actual time from when conveyance of the paper from the second supply means to the image formation section is initiated to when the paper arrives at a position at which acceleration is possible.
Furthermore, the position or the timing at which the conveyance velocity of the reversed paper is decelerated may be determined by measuring an actual time required for the paper to arrive at a position where deceleration is possible from the position where reversal of the paper started.
According to the second aspect, there is provided an image forming apparatus having a paper waiting position where a sheet of paper to be supplied to an image formation section is caused to wait temporarily and a paper reversing means for reversing said paper with an image formed on one side thereof, said image forming apparatus comprising a paper supply means for supply paper from said paper waiting position to said image formation section; a first paper conveying means for conveying the paper with the image formed on one side thereof from said image formation section to said paper reversing means; a second paper conveying means for conveying the paper reversed by said paper reversing means to said paper waiting position; a controlling means for controlling a paper conveying velocity so that said paper is accelerated at a predetermined timing during being conveyed by said first paper conveying means and the reversed paper is decelerated at another predetermined timing during being conveyed by said second paper conveying means; and a correction means for correcting said timing of acceleration and deceleration of said paper.
The inventor of the present invention arrived at the invention by realizing that, in the image forming apparatus employing the non-stacked circulatory conveyance method for forming images on both sides of a sheet of paper, fluctuations of time required for the paper to arrive at the registration rollers for the purpose of formation of an image on the back side of the paper can be compensated by correcting the position or the timing at which the reversed paper is accelerated and/or decelerated in the circulatory conveyance path. As a result, it is not necessary to control the operation of the registration rollers at fixed intervals determined by CPM when forming an image on the back side of the paper.